1. Field of the Invention
The present invention relates generally to acoustic signals and particularly to a method and system for enhancing monophonic and stereophonic acoustic signals.
2. Description of Related Art
It is impossible to achieve the same degree of channel separation in a typical two loud-speaker stereo system that is possible with a pair of headphones. In such a stereo system, acoustic signals arriving at a listener's ear from the left and right loud-speakers which are in phase tend to add, while those which are out of phase tend to cancel one another. This phenomena, known as speaker crosstalk, degrades the perceived spatial and directional qualities of the acoustic image. Further, since speaker crosstalk is a function of the geometry of the interfering wavefronts resulting from the intersection of the left and right acoustic signals, the effects of speakers crosstalk are dependent upon the location of the listener relative to the positions of the left and right speakers. That is, the effects of crosstalk as perceived at one location may be different from those perceived at another location. This positional dependence of crosstalk gives rise to the so-called "dead spots" and "sweet spots" a listener experiences when moving across a listening area.
It is theoretically possible to cancel crosstalk by enhancing the stereo signals as a function of the particular positions of the speakers and the dynamic position of the listener. In practice, however, such cancellation is impossible to achieve since the particular arrangement of a listener's speakers and the dynamic position of the listener cannot be predicted. Numerous stereo enhancement systems have been disclosed recently which attempt to compensate for this positional dependence of crosstalk by enhancing the (L-R), or difference, component and the (L+R), or sum, component of the stereo signals. Such systems, however, are relatively complex and expensive to implement.
Further, many of the conventional stereo enhancement systems fail to effectively address the monophonic aspects of stereo signals. For instance, it is desirable in a stereo enhancement system to retain compatibility with monophonic receivers, that is, receivers which receive only the modified sum (L+R) component of the stereo signal. Receiving only the modified sum component without the ability to extract the spatial effects encoded into the difference signal results in an undesirable degradation of the original monophonic acoustic image.
In addition, since many of the presently broadcast and recorded acoustic images include both stereo and monophonic sources, it is also desirable for a stereo enhancement system to not only spatially enhance monophonic acoustic images but also to have the ability to smoothly and automatically transition between stereo signal enhancement and monophonic signal enhancement.